1. Field of the Invention
The present invention relates to a coaxial probe which can be introduced into a living tissue or the like to carry out heat-treatment using microwaves.
2. Description of the Related Art
Recently, for treatment of diseases such as malignant tumors or the like, several therapies using electromagnetic waves have been developed. One of the therapies is a generally known coagulation therapy which is carried out using a coaxial probe.
According to the coagulation therapy, the coaxial probe is inserted directly into the affected part of a patient. The affected part is heated directly using electromagnetic waves radiated from the coaxial probe, so that the tissue in the affected part is coagulated so as to be killed off.
Referring to methods of inserting coaxial probes into affected parts, e.g., for livers, puncturing in an incised abdomen, puncturing through the skin, insertion into the guide probe of a throracoscope or caparoscope, and so forth are known. In the above-described coagulation therapy using the coaxial probe, the incised part is relatively small, and the time required for the treatment is relatively short. Thus, advantageously, when a patient is operated, the burden to him or her can be reduced.
FIGS. 5A and 5B show the structure of a known coaxial probe (e.g., see Japanese Unexamined Patent Application Publication No. 7-275247). FIG. 5A is a longitudinal cross-sectional view of the coaxial probe 100 inserted into a schematically-shown organ such as a liver or the like. FIG. 5B is a cross-sectional view of the coaxial probe taken in the plane perpendicular to the longitudinal direction of the coaxial probe 100. As shown in FIGS. 5A and 5B, a dielectric 2 is interposed between an inner conductor 1 and an outer conductor 3. The outer conductor 3 and the inner conductor 1 are electrically connected to each other via the top end portions thereof. A slit S is formed in a part of the outer conductor 3.
FIG. 7 is a longitudinal cross-sectional view showing the structure of the top-end portion of the coaxial probe described in Patent Document 1. A microwave antenna portion 67 (the slit S shown in FIG. 5A) is provided near the end-portion of a coaxial probe cable 66. The coaxial cable 66 is inserted into a sleeve which is divided into an insertion portion 61 and a top-end portion 62. A top-end chip 63 is fixed to the end of the top-end portion 62. Thus, the coaxial probe which can be used in puncturing is formed.
Specifically, the sizes (unit: mm) of the known coaxial probe are entered in FIGS. 5A and 5B. The coaxial probe having the above-described structure and size was simulated by a computer. The reflection coefficient was 0.65. In other words, about 65% of input signals were reflected toward the input side. Thus, the known coaxial probe has problems in that the radiation efficiency with respect to electromagnetic waves is low.
Moreover, the radiation pattern of the coaxial probe was measured. FIG. 6 shows the results. In FIG. 6, the position (unit: mm) of the coaxial probe is plotted as the abscissa, in which the top-end position of the coaxial probe is taken as zero. The distance (unit: mm) in the radial direction is plotted as the ordinate. SAR (Specific Absorption Rate) is shown in graded concentrations. SAR represents the energy-quantity of an electromagnetic wave absorbed in a living body, and is used for the evaluation of the energy quantity. SAR represents the energy per unit time absorbed in unit mass in a unit of W/kg. In FIG. 6, the one step of the graded concentrations is equivalent to 2.5 dB. As seen in FIG. 6, SAR becomes high in the vicinity of the slit S. Thus, the affected part is heated in the vicinity of the slit S.
In the case of the known coaxial probe, the slit S is provided at a position that is spaced from the top end of the coaxial probe 100 (10 mm in the example shown in FIGS. 5A and 5B). Thus, the known coaxial probe cannot be conveniently used. In particular, the coaxial probe is partially passed through an affected part to be heated so that the slit of the coaxial probe is positioned in the center of the affected part. Thus, the known coaxial probe has problems in that the insertion degree (the degree at which the coaxial probe is inserted into a normal tissue) of the coaxial probe increases or the area of an affected part which can be treated becomes small.
However, if the slit is positioned near the top end of the coaxial probe improperly, the radiation efficiency cannot be enhanced, since the reflection coefficient decreases as described above.
Moreover, regarding the known coaxial probe in which the top-end chip 63 is fixed to the top-end of the coaxial probe, so that the puncturing of a living body can be realized as shown in FIG. 7, the top-end chip is large in size, and probably, the chip perforates an affected part, thus damaging a normal tissue.